/*
 * Copyright 2004-2005 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package net.sf.cindy.util.queue;

/**
 * FIFO Queue. A Queue does not accept null elements. Implementations 
 * should keep all methods are thread-safe.
 * 
 * @author Roger Chen
 */
public interface Queue {

    /**
     * add object to the queue. The object can't be null.
     * 
     * @param obj
     * 		the object
     */
    void push(Object obj);

    /**
     * Get the header object and delete it from the queue.
     * 
     * @return
     *		the object
     */
    Object pop();

    /**
     * Get the header object.
     * 
     * @return
     * 		the object
     */
    Object peek();

    /**
     * Clear the queue.
     */
    void clear();

    /**
     * Get the queue's size.
     * 
     * @return 
     * 		queue's size
     */
    int size();

    /**
     * The queue is empty.
     * 
     * @return
     * 		is empty
     */
    boolean isEmpty();

}